At the base, in the perfectibility of man, as I discussed elsewhere in this subthread: https://news.ycombinator.com/item?id=13055322 Pretty much everything flows from that.
To take the best personal example I have, in my lifetime, since I became politically and socially aware in the very early '70s, "racist" meant something, something effectively concrete, something real and observable. Now can mean something as anodyne as disagreeing with an opinion or position of Obama's.
This is not an accurate understanding.
But it is a useful point for the conversation to reach. If you are not willing to evaluate the accuracy of that belief, you also won't give consideration to other arguments that contradict it.